The purpose of this study is to examine the neural basis of response inhibition, working memory, and sustained attention in adolescents and adults with Attention-Deficit/Hyperactivity Disorder (ADHD), with particular emphasis on quantifying the effects of methylphenidate (i.e., treatment with psychostimulants) on neural function. Participants will undergo electrophysiological measurement of brain function during laboratory cognitive tasks. This research is aimed to develop a better understanding of how ADHD neural dysfunction relates to clinical presentation and medication response during the transition from adolescence to adulthood. The specific aims and hypotheses are: